1. Field of the Invention
The present invention relates to a dielectric filter for selectively filtering a high-frequency signal having a desired frequency mainly used in a base station for a mobile communication system such as car telephones and portable telephones. More particularly, the present invention relates to a dielectric notch filter. The present invention also relates to a dielectric resonator constituting the dielectric filter.
2. Description of the Related Art
In recent years, as the development of the mobile communication system such as car telephones, a notch filter using a dielectric resonator is increasingly demanded.
Hereinafter, an exemplary conventional dielectric notch filter will be described with reference to figures. FIGS. 24A and 24B are external views of a conventional dielectric notch filter. FIG. 24A is a top view and FIG. 24B is a side view. In these figures, the dielectric notch filter includes cylindrical metal cavities 2401, a base member 2402, tuning members 2403, and input/output terminals 2404. The notch filter shown in FIG. 24 has five resonators. A transmission line is formed in the base member 2402 and electromagnetically coupled with the respective dielectric resonators, so as to constitute the notch filter. FIG. 25 shows the inside of a dielectric resonator used in the conventional dielectric notch filter shown in FIG. 24 in a simplified manner. In the metal cavity 2401, a dielectric block 2501 and a coupling loop 2502 for electromagnetic coupling are provided. FIG. 26 is a cross-sectional view showing an adjusting mechanism for adjusting the degree of electromagnetic coupling in the conventional dielectric resonator. As shown in FIG. 26, the adjusting mechanism includes a supporting member 2 for supporting the dielectric block 2501, a loop 4a of the coupling loop 2502, a ground part 4b of the coupling loop 2502, a handle 4c for rotating the whole coupling loop 2502, and a pole 5 of the coupling loop 2502. The pole 5 is composed of a center conductor 5a and an insulator 5b. The base member 2402 includes a transmission line 7 serving as an inner conductor and outer conductors 8. The transmission line 7 is supported by a supporting member 9 which is an insulator. In general, the dielectric block 2501 is formed integrally with and supported by the supporting member 2 using glass with a low melting point. The operation principle of the conventional dielectric resonator having the above-described construction will be described below. When the dielectric block 2501 and the coupling loop 2502 are held in the metal cavity 2401 and the transmission line 7 is connected thereto, an electromagnetic field is produced in the cavity 2401. Thus, the conventional dielectric resonator has a resonance frequency corresponding to a resonant mode. The degree of electromagnetic coupling of the dielectric resonator is a critical parameter for determining the electric characteristic of the dielectric resonator. The degree of electromagnetic coupling is determined depending on the number of lines of magnetic force across the cross section of the coupling loop 2502. That is, according to the conventional technique, the coupling loop 2502 is mechanically rotated by the handle 4c and hence the effective cross-sectional area is varied, so that the number of lines of magnetic force across the coupling loop 2502 is adjusted.
In order to match the impedance of the dielectric resonator, the electric length of the coupling loop is precisely adjusted to be an odd-integer multiple of a quarter wavelength.
However, the above-described prior art has the following drawbacks.
(1) A complicated mechanism for mechanically rotating the coupling loop is required, and hence the number of components required is increased.
(2) The means for impedance matching is limited, and the size of the coupling loop is greatly increased for lower frequencies. Also, since the coupling loop is small for higher frequencies, it is impossible to attain a higher degree of coupling.
(3) In principle, the range of frequencies in which the impedance matching can be achieved is narrow.
(4) In order to melt the glass for adhesion, a heating treatment to the dielectric member is required. The adhesive strength of glass is low, and the mechanical reliability is poor.
As a result, the following problems arise.
(1) The coupling loop is easily rotated due to vibration and impact, so that the degree of electromagnetic coupling is varied.
(2) The production process is complicated.
(3) The production cost is increased.
The dielectric notch filter of this invention includes: a transmission line for transmitting a high-frequency signal; an input terminal and an output terminal provided at both ends of the transmission line; a ground conductor for supplying a ground potential; and a dielectric resonator connected to the ground conductor and the transmission line, wherein the dielectric notch filter further comprises impedance matching means connected to the ground conductor and the transmission line in parallel with the dielectric resonator, and the dielectric resonator includes: a cavity connected to the ground conductor; a dielectric block provided in the cavity; a coupling device coupled with an electromagnetic field produced in the cavity; and a coupling adjusting line for connecting the coupling device to the transmission line and for adjusting the degree of electromagnetic coupling.
In one embodiment of the invention, the degree of electromagnetic coupling is adjusted by an electrical length of the coupling adjusting line.
In another embodiment of the invention, an impedance value of the impedance matching means is adjusted in accordance with an electrical length of the coupling adjusting line.
In another embodiment of the invention, the coupling adjusting line is formed of a TEM mode transmission line, and the degree of electromagnetic coupling is adjusted by a dielectric material inserted between the TEM mode transmission line and the ground conductor.
In another embodiment of the invention, the impedance matching means is an inductor. The inductor may be an air-core coil.
In another embodiment of the invention, the impedance matching means is a capacitor.
In another embodiment of the invention, the impedance matching means is a stub.
In another embodiment of the invention, the coupling adjusting line or the impedance matching means is formed by a conductor pattern provided in a dielectric substrate.
According to another aspect of the invention, the dielectric notch filter includes: a transmission line for transmitting a high-frequency signal; an input terminal and an output terminal provided at both ends of the transmission line; a ground conductor for supplying a ground potential; and a plurality of dielectric resonators connected to the ground conductor and the transmission line, wherein the dielectric notch filter further comprises a plurality of impedance matching means connected to the ground conductor and the transmission line in parallel with the plurality of dielectric resonators, and each of the dielectric resonators includes: a cavity connected to the ground conductor; a dielectric block provided in the cavity; a coupling device coupled with an electromagnetic field produced in the cavity; and a coupling adjusting line for connecting the coupling device to the transmission line and for adjusting the degree of electromagnetic coupling, resonance frequencies of the respective plurality of dielectric resonators being distributed symmetrically with respect to a filter center frequency.
In one embodiment of the invention, the plurality of dielectric resonators are first to fifth dielectric resonators, the first to fifth dielectric resonators being arranged in a direction from the input terminal to the output terminal, and the first to fifth dielectric resonators have resonance frequencies F1 to F5, respectively, the resonance frequencies F1 to F5 satisfying conditions of:       F4    =          fo      +      df2            F2    =          fo      +      df1            F1    =    fo        F5    =          fo      -      df1            F3    =          fo      -      df2      
where 0 less than df1 less than df2, and fo denotes the filter center frequency.
In another embodiment of the invention, transmission lines between the first and the second dielectric resonators and between the fourth and the fifth dielectric resonators have electrical lengths larger than xcex/4xc3x97(2mxe2x88x921) and smaller than xcex/4xc3x97(2mxe2x88x921)+xcex/8, transmission lines between the second and the third dielectric resonators and between the third and the fourth dielectric resonators have electrical lengths larger than xcex/4xc3x97(2mxe2x88x921)xe2x88x92xcex/8 and smaller than xcex/4xc3x97(2mxe2x88x921), where xcex denotes a wavelength, and m is a natural number.
According to another aspect of the invention, a dielectric resonator is provided. The dielectric resonator includes: a cavity; a dielectric block fixed in the cavity; and a coupling device coupled with an electromagnetic field produced in the cavity, wherein a through hole is formed in the dielectric block, a fixing shaft formed of a dielectric material is allowed to pass through the through hole, and one end of the fixing shaft is fixed to the cavity by a presser member.
In one embodiment of the invention, the dielectric block resonates in a TE mode, and the through hole is provided in parallel to a propagation axis direction.
In another embodiment of the invention, the fixing shaft is threaded, and the presser member is a resin nut.
In another embodiment of the invention, the resin nut is provided with a protrusion which fits in the through hole.
In another embodiment of the invention, a resin washer having a protrusion which fits in the through hole is sandwiched between the resin nut and the dielectric block.
In another embodiment of the invention, a diameter of the through hole is larger than a diameter of the fixing shaft, and a gap is provided between the dielectric block and the fixing shaft.
In another embodiment of the invention, a supporting member having a through hole is allowed to pass through the fixing shaft, and the dielectric block is supported by the supporting member.
According to another aspect of the invention, the dielectric resonator includes: a bolt formed of a dielectric material; a bolt pressing plate having a through hole; a supporting member having a through hole; a dielectric block having a through hole; and a cavity, wherein the bolt is allowed to pass through the through holes of the bolt pressing plate, the supporting member, and the dielectric block in this order, and fastened with a nut, thereby constituting a resonator unit, the resonator unit being fixed to the cavity.
In one embodiment of the invention, a portion of the cavity at which the resonator unit is fixed has a thickness larger than a thickness of a head portion of the bolt, and an opening is provided for allowing the head portion of the bolt to pass, the opening being closed by the bolt pressing plate.
According to another aspect of the invention, the dielectric resonator includes: a dielectric block having one of a columnar shape or a cylindrical shape and having a diameter d and a height h; and a rectangular parallelepiped metal cavity having a width W, a depth D, and a height H, wherein the dielectric block is held in a center portion of the metal cavity, and a ratio of the depth D to the diameter d is in the range of 1.3 to 2.0, a ratio of the width W to the diameter d is in the range of 2.0 to 4.0, and a ratio of the width W to the depth D is in the range of 1.2 to 2.5.
In one embodiment of the invention, at least one coupling loop or at least one coupling probe is provided in the metal cavity between the dielectric block and at least one of two faces of the metal cavity defined by the width W and the height H.
In another embodiment of the invention, at least one coupling loop or at least one coupling probe is provided in the metal cavity between the dielectric block and at least one of two faces of the metal cavity defined by the depth D and the height H.
In another embodiment of the invention, the dielectric block is surrounded by a metal strap in a circumferential direction thereof, whereby the metal strap has top and bottom openings, and both ends of the metal strap are jointed by a method selected from welding, soldering, silver soldering and tabling, resulting in the metal cavity.
According to another aspect of the invention, a dielectric filter is provided in which dielectric resonators are arranged and fixed in a direction of the depth D, and the dielectric resonators are electrically connected to each other.
According to another aspect of the invention, the dielectric filter includes: N dielectric blocks each having one of a columnar shape or a cylindrical shape and having a diameter d and a height h, N being an integer of 2 or more; a single metal case having a rectangular parallelepiped shape and having a width W, a depth Nxc3x97D, and a height H; and (Nxe2x88x921) metal partitions each having a width W and a height H, wherein the metal case is divided by the metal partitions into substantially equal portions along a direction of the depth Nxc3x97D, thereby forming N rectangular parallelepiped cavities having the width W, a depth D, and the height H, and the dielectric blocks are held in the center portions of the cavities, respectively, a ratio of the depth D to the diameter d being in the range of 1.3 to 2.0, a ratio of the width W to the diameter d being in the range of 2.0 to 4.0, and a ratio of the width W to the depth D being in the range of 1.2 to 2.5.
According to another aspect of the invention, a dielectric resonator is provided. The dielectric resonator includes: a cavity having a first threaded hole; a dielectric block provided in the cavity; a coupling device coupled with an electromagnetic field produced in the cavity; a frequency tuning member having a screw portion which is spirally engaged with the first threaded hole of the cavity, a distance between the dielectric block and the frequency tuning member being changed by rotating the frequency tuning member, for tuning a resonance frequency of the cavity depending on the distance; fixing means for fixing a relative positional relationship between the frequency tuning member and the cavity, wherein the fixing means fixes the cavity and prevents the frequency tuning member from rotating due to a frictional force caused between the first threaded hole of the cavity and the screw portion of the frequency tuning member.
In one embodiment of the invention, the fixing means includes a lock nut and a fixing screw, the lock nut having a second threaded hole which is spirally engaged with the screw portion of the frequency tuning member and a through hole through which the fixing screw is passed, the cavity having a third threaded hole which is spirally engaged with the fixing screw, and the fixing means applies a force in a direction in which the lock nut and the cavity come closer to each other by tightening the fixing screw.
In another embodiment of the invention, the fixing means has a lock nut and a fixing screw, the lock nut having a fourth threaded hole which is spirally engaged with the screw portion of the frequency tuning member and a fifth threaded hole which is spirally engaged with the fixing screw, and the fixing means applies a force in a direction in which the lock nut and the cavity become are moved away from each other by tightening the fixing screw.
Thus, the invention described herein makes possible the advantages of (1) providing a dielectric notch filter having a simplified adjusting mechanism for adjusting the degree of coupling as compared with the conventional dielectric notch filter in which the degree of electromagnetic coupling is easily adjusted, (2) providing a method for supporting a sturdy dielectric block which is easily produced with lower power loss, (3) providing a compact and high-performance cavity, (4) providing a tuning mechanism which is constructed with a smaller number of components, and (5) providing steep notch filter characteristics.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.